US President Donald Trump has defended enquiring about his presidential rival Joe Biden during a call with the Ukrainian President Volodymyr Zelensky.
Mr Trump denied pressuring Mr Zelensky to look into unsubstantiated corruption allegations against Mr Biden.
But accusations that Mr Trump abused his power in raising Mr Biden with a foreign leader have pushed more Democrats towards raising impeachment.
A top Democrat said impeaching the president now "may be the only remedy".
Democrats have accused Mr Trump of threatening to withhold a $250m (£200m) aid payment to Ukraine as a means to pressure Mr Zelensky to investigate Mr Biden and his son Hunter.
Mr Trump and his allies have claimed that Mr Biden encouraged the firing of Ukraine's top prosecutor because the prosecutor was investigating a company that employed Hunter Biden - a claim which has never been substantiated.
"Of course it's an impeachable offence," Democratic congressman Jim Himes told CNN of the president's alleged attempt to pressure Ukraine.
"I can't tell you that the House will move into impeachment mode right away, but this really ups the ante," Mr Himes said.
President Trump has not provided any evidence of corruption involving Mr Biden or his son, Hunter, and the former vice-president denies any wrongdoing.
Pressure is growing on Mr Trump to release a transcript of his phone call with Mr Zelensky.
What's the background?
Mr Trump and his allies have been suggesting that Mr Biden, as Barack Obama's vice-president, encouraged the firing of Ukraine's top prosecutor because he had been investigating Ukrainian natural gas company Burisma, which employed Hunter Biden.
Hunter Biden became a director at Burisma in 2014, while his father held a key role in US policy towards Ukraine.
Mr Biden is currently frontrunner to be the Democrat to take on Mr Trump in the November 2020 presidential election.
US media reported last week that an intelligence whistleblower had filed a complaint over Mr Trump's alleged promise to a foreign leader during a call.
The US president pressured his Ukrainian counterpart to launch corruption investigations into the Bidens, according to US media reports.
It emerged last week that the Trump administration was blocking the whistleblower complaint from being handed over to Congress, despite the intelligence inspector general judging it to be "urgent".
Under US law, if a complaint is considered to be of "urgent concern", and if the inspector general considers the complaint to be "credible", then the department head is expected to share the information with Congress within seven days.
The controversial phone call between the two leaders coincided with the White House's decision to temporarily freeze up to $250m in military aid to Ukraine.
The decision to suspend the aid was made before the call on 25 July, US media report, and there has been no indication the package was discussed on that particular phone conversation.
Following bipartisan criticism of the aid delay, the administration approved the $250m for Ukraine this month.
Mr Trump's lawyer Rudy Giuliani told Fox Business on Monday that he could not say "100%" that the president did not threaten to withhold military aid to Ukraine.
What did Trump say?
Arriving on Monday at the United Nations General Assembly in New York City, Mr Trump insisted there had been no wrongdoing on his call with Mr Zelensky.
"What Biden did was wrong," the Republican president added.
Asked why his administration had delayed the military aid to Ukraine, Mr Trump added: "Why would you give money to a country that you think is corrupt?"
Mr Trump told reporters on Sunday that his call with Mr Zelensky had been to congratulate him on his recent election.
But Mr Trump also said during the conversation he mentioned corruption and "largely the fact that we don't want our people, like Vice-President Biden and his son, creating to the corruption already in the Ukraine".
He insisted he had done "absolutely nothing wrong". Mr Trump has previously accused the as-yet-unnamed whistleblower of being "partisan" and said he knew all his phone calls to foreign leaders were listened to by US agencies.
While Mr Trump suggested a transcript could be released, senior Trump administration officials said it would be inappropriate for private conversations between world leaders to be made public.
The Wall Street Journal has quoted sources as saying Mr Trump had urged Mr Zelensky about eight times to work with his lawyer Rudy Giuliani on an investigation into Hunter Biden, but had not offered anything in return.
Democrats have said that if Mr Trump asked Mr Zelensky to investigate Mr Biden, it is tantamount to promoting foreign interference in the 2020 election.
The US president is due to meet his Ukrainian counterpart on Wednesday, among several other foreign leaders at the UN General Assembly.
What are the Biden claims?
When Hunter Biden joined Burisma in 2014, questions were raised about a potential conflict of interest for his father.
Ukraine was undergoing a political transition after its pro-Russia president was forced out of office, and the elder Biden was making frequent trips to the country.
In 2016, Joe Biden pushed the Ukrainian government to fire its top prosecutor, Viktor Shokin, whose office had been investigating the oligarch owner of Burisma.
In a speech last year at a think tank Mr Biden boasted of having threatened to withhold a billion-dollar loan guarantee to successfully force Mr Shokin out.
"I looked at them and said 'I'm leaving in six hours: if the prosecutor is not fired, you're not getting the money'," he said.
Mr Trump and his allies accuse Mr Biden of having acted to protect his son. However, several western governments and major financers of Ukraine's government also wanted Mr Shokin dismissed because he was seen as a barrier to anti-corruption efforts.
Mr Shokin had also shown little appetite for pursuing Burisma.
Joe Biden has said he has never spoken to his son about his business dealings.
What's being said about impeachment?
Democrats have been split on whether they should seek to impeach Mr Trump over alleged wrongdoing.
However, this latest story appears to have pushed some senior Democrats closer to backing such a move.
Adam Schiff, the Democratic chairman of the House Intelligence Committee, said impeachment "may be the only remedy".
Mr Schiff has previously resisted calls among rank-and-file Democrats that they attempt to remove the president from office.
House Speaker Nancy Pelosi said that if the whistleblower complaint was not turned over to Congress the Trump administration would "be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation".
Ms Pelosi, like Mr Schiff, has thus far resisted demands for impeachment.
Mr Trump was asked by a journalist on Monday how seriously he was taking the threat of impeachment, and he replied: "Not at all seriously."